Adding a bathroom in the attic, deciphering the current plumbing

I am adding a bathroom in my attic, and when I opened up the wall it looked like the plumbing was already set for a bathroom, including a separate vent line and 2 capped off copper water lines (see picture). The fittings on the waste line are too high, but I am wondering if anyone could tell me if I can cut the line low enough and tie in a PVC Y for all my plumbing. Will someone with years of plumbing experience please give me some advice! Thanks

I am also wondering about the two copper water lines. They are not live, and since that was put in the entire plumbing system in the house has been redone with pex. I am wondering if I could save myself sometime and try to find the bottom of those lines and tie into them? How would I go about finding the other ends? Or maybe it would be better to just run new pex all the way up myself? Let me know what you think.

Agree with hj. Cast iron is not such an easy DIY project. But of course you can redo all this in plastic with no hubs and banded couplings . The vent tie in would need to be 6" above the rim of any sink you put in. From the picture, it does not look like its that high. Since the access is easy, this can all be fixed up. But when you cut into that CI, you might disturb the roof jack, so that's one more thing to watch out for.l

I have actually cut into CI before and tied in a Y for a washer and dryer drain. I think I will be able to do the same here without too much trouble, I just use a angle grinder and saw right through it.

What I am wondering is, do I need to tie in my PVC Y just below the floor, so its lower than my toilet and shower drain, or do I need to run my pipe down below the 2nd level fixtures? From my understanding what I need to know is how the 2nd level toilet is vented, and if it is vented separate from the drain stack then I can tie in just below the floor, but if it is vented just from the main stack then I need to go lower. Is this understanding correct? Is there a way to know how the 2nd level toilet is vented without tearing up my wall and floor?

NO. The assumption would be that if the system was designed to have that toilet installed, the second floor would be vented to accomodate it. But you would still have to know for sure and there is no way to tell without seeing the piping.

Would it be safe to just go with the assumption and tie it in under the attic floor. What would happen if the toilet wasnt properly vented and my assumption was wrong. Would it just go against code, or would it not function properly?

Tie it in under the attic floor.Plan on getting a SNAP CUTTER (rent for an hour or 2.)
Makes very short work of the cast and doesn't get that metal dust all over.
Just a safer procedure all the way around.
If all the plumbing below is tied in through that 2" coming up plan on cutting that ,raising it up and you can "stack vent" (by local code MN) with in 'dirty arm' distances.
4' for your toilet
2'6" on 1 1/4
3'6" on 1 1/2
5' on 2"
MN code for all this can be found FREE of charge. on the MN state page.(I know,your likely not in MN.)
Cutting out all that heavy old cast iron will give your house better gas mileage! ;-)

Actually I am located in Minnesota as well, what a coincidence. I think I will rent a snapper for under the floor and that should go pretty quick. For the vent I didn't really understand what you meant. I was planning on snapping a section out of the same stack again, a couple of feet higher than where the vent is tied in, and then cutting the vent and adding in a T so it would be high enough. I don't know what a "dirty arm" is.

I was also thinking about cutting out that entire section of pipe and replacing it will PVC. Of course I want to do what will be that fastest and cheapest. Thanks for your help.